The use of non-motion-based cues to pre-programme the timing of predictive velocity reversal in human smooth pursuit.

Human smooth pursuit eye movements are principally driven by visual feedback and cannot normally be initiated at will. However, when tracking periodic motion, smooth eye movements reverse direction prior to target reversal, driven by anticipation, not visual feedback. Here, we investigate cognitive control over such eye reversals. Target stimuli were discrete double ramps-constant speed (30 degrees /s) rightwards followed by similar leftward movement, reversal time ranging from 420 ms to 840 ms. Three experimental conditions were examined. In the precued condition, double ramps of randomised reversal time were presented. Prior to ramp presentation audio precues were given with an interval indicating start and reversal time of the unseen, upcoming double ramp. Subjects were able to use these cues to voluntarily control timing of anticipatory eye reversal, so that when occasional false precues gave underestimates of target reversal time, eye reversal occurred before target reversal. Precued eye reversal times were comparable to those in a second, predictable condition, in which double ramps with identical reversal time were given repeatedly without precues. In contrast, reversal occurred much later in a third, reactive condition, also without precues, when unexpected early target reversals occurred sporadically within a series having identical, predictable reversal times. The findings provide evidence that timing of anticipatory smooth eye movement, both at the start of the double-ramp and at its reversal, can be independently controlled at will using non-motion-based timing cues.